1. Field of the Invention
This invention relates to a metal vapor laser device.
2. Description of the Prior Art
A metal vapor laser device generates laser oscillations making use of vapor of a working metal which is enclosed in a laser tube together with a carrier gas.
An exemplary one of metal vapor laser devices which are practically used at present is a so-called positive column type metal vapor laser device which generates laser oscillations making use of a positive column portion of a discharge. More particularly, a positive column type He-Cd laser device is known wherein helium is used as a carrier gas and cadmium is used as a working metal.
Such a He-Cd laser device can continuously oscillate, for example, an ultraviolet ray having a wavelength of 325 nm as well as a visible ray having a wavelength of 442 nm in a short wavelength zone. Due to the characteristic, the demand for such He-Cd lasers has increased in recent years, and He-Cd laser devices are used in various fields as light sources, for example, for laser printers, holography, photo-plotters, color scanners and so on.
The output laser power of a metal vapor laser device depends upon a pressure of vapor of a working metal within a metal vapor laser tube of the laser device. Accordingly, in order to obtain a stable laser beam, commonly it is necessary to maintain the pressure of metal vapor within the metal vapor laser tube constant.
An exemplary one of conventional technical means for maintaining the pressure of metal vapor within a metal vapor laser tube constant takes advantage of the fact that, if the pressure of metal vapor within the metal vapor laser tube drops, then the inter-electrode voltage, that is, the voltage across the anode and the cathode, of the metal vapor laser tube rises. According to the conventional means, a variation of the inter-electrode voltage is detected, and such a detection signal is fed back to control the power supply to a heater for heating a working metal within a metal tank of the metal vapor laser tube.
The conventional means, however, has a drawback that the pressure of metal vapor within the laser tube cannot be maintained sufficiently constant because operation of the heater is feedback controlled in response to an inter-electrode voltage of the metal vapor laser tube. In particular, if the atmospheric temperature of the metal vapor laser tube varies suddenly, then a significantly long interval of time is required until such a variation in temperature is detected as an inter-electrode voltage because the speed of response of a variation in pressure of metal vapor to a variation of the atmospheric temperature is low. Accordingly, the means of feedback controlling the operation of the heater in response to a detected variation of the inter-electrode voltage of the metal vapor laser tube will present a considerable delay in time until the variation in pressure of metal vapor caused by a variation in temperature is compensated for. After all, quick control is difficult, and a stable laser beam cannot be produced.